This invention relates to a capacitance probe for level detection and, in particular, to a capacitance probe useful for detecting the level of fly ash in a receiver vessel.
Fly ash which is precipitated from exhaust gases by Cottrell precipitators and the like is collected in receiver vessels having inlet and outlet valves to permit sequential receiving and discharging operations in response to the level of solids collected in the vessels. Fly ash has a low dielectric constant and readily coats capacitance probes and other level detectors, and thus presents a difficult application for capacitance level measurement.
One attempt to provide a stable and sensitive capacitance level detector is that shown in U.S. Pat. No. 3,934,475. The capacitance probe described and claimed in this patent avoids direct contact of the probe and fly ash by location of a capacitance plate on the outside of an insulating window in the sidewall of the vessel. This capacitance level sensor generally lacks sensitivity and stability because of filming or coating of the insulating window with the fly ash.